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Black Hole
Special Relativity
• Einstein’s special theory of relativity has two parts.
– All objects moving at constant velocity have the same laws
of physics
– The speed of light is constant for all observers
The pendulum will
swing at the same rate if
the car is moving or not.
General Relativity
• If nothing travels faster than light, how does gravity affect
objects?
• General relativity showed that acceleration acts just like
gravity on objects.
On Earth, with
gravity, objects in
the rocket have 1 g
pulling down.
In space, with
no gravity, the
engines can
create a thrust
of 1 g pulling
to the floor.
Bending Light
• Photons, which transmit light, are massless.
– Photon energy has an equivalent mass
– Photons affected by gravity, which bends the light
• We picture space as curved by gravity, and light follows
the curves like a lens.
Pure Gravity
• In a neutron star, degenerate neutrons resist gravity.
• For Mns greater than 3 M the neutron star collapses.
• There is nothing left to oppose gravity and the mass
collapses to an infinitesimal point.
Trapped Light
• The gravitational force increases as the distance decreases.
– Escape velocity also increases
– Escape velocity can exceed the speed of light
– No light or matter escapes
• This is called a black hole.
Photon
deflected
Photon
trapped
Observing Radiation
• Matter which reaches the
event horizon is trapped.
• The acceleration causes
radiation before matter is
lost.
– All parts of spectrum
Black Hole Mass
• Black holes can keep accumulating mass.
– Stars orbit around according to Kepler’s laws
– Measure orbit and calculate mass
• Some black holes have the mass of over a million suns.